Photoelectric tube



y 1940. s. B.IINGRAM 2,208,129

PHOTOELECTRIC TUBE Filed Dec. 21, 1937 lNl/EN TOR y S. B. INGRAM T TORNEV atente duty 3. i

n. t" r7 Telephone Laboratories, Incorporated, New

York, N. Y a corporation or New York 'Application December 21, 1937, Serial No. 180,939

6 Glaims.

This invention relates to photoelectric tubes and more particularly to the electrode structure and method of sensitizing the cathodes of such tubes.

An object of the invention is to provide an improved form of double anode photoelectric tube.

Another object is to provide an improved process of sensitizing such a photoelectric tube.

In an example of practice illustrative of this invention, a cathode and two anodes are supported from a stem within an evacuated container. The cathode comprises a plate formed into two'hollow quarter circular cylinders joined along one edge and curved away from one another so that no portion of either concave surface faces the other concave surface. The anodes are thin wires positioned respectively in front of said concave surfaces and parallel thereto. A sheet of insulating material, such as mica, bisects the space between the convex surfaces of said cathode. From the separated edges of the quarter cylinders two wings extend toward one another. These wings are secured to cathode support wires. Another sheet of mica extends between the separated edges of the quarter cylinders and contacts with the inner surfaces of the wings.

In a preferred method of sensitizing the concave surfaces an electric discharge in oxygen is produced by impressing a potential between the cathode plate and the anodes, the anodes being connected in parallel. The mica sheets facilitate the oxidation of the concave surfaces of the cathode by causing the discharge to take place substantially entirely between the anodes and the concave surfaces. Since these surfaces are preferably silver surfaces, a uniform layer of silver oxide is formed which when treated with caesium vapor under suitable temperature conditions results in a highly eiiicient photoemissive surface.

A marked improvement in the manufacture of this type of photoelectric tube is efiected by the use of these mica sheets particularly by the use of the sheet which bisects the space between the convex surfaces of the quarter cylinders. Without this sheet of insulating material the discharge tends to concentrate on the enclosed surfaces of the cathode including these convex Photoelectric tubes according to this invention are particularly useful in the projection of sound from the so-called push-pull sound records. When used for this purpose the anodes are connected separately to the sound circuit. If it is desired to project sound from ordinary sound records the anodes may be connected together. The light beam can then be projected on the whole cathode surface without loss of efficiency because there is no separation between the two concave surfaces of the cathode.

This invention will now be described more in detail having reference to the accompanying drawing.

Fig. 1 shows the structure of a completed photoelectric tube according to this invention.

Fig. 2 is a cross-section of the tube of Fig. l perpendicular to the axes of and through the electrodes looking away from the stem.

Fig. 3 is an exploded View of the cathode and insulating plates.

Fig. 4. illustrates a pumping station suitable for processing four tubes concurrently.

Fig. 5 is a schematic electric circuit used in processing such tubes.

The tube illustrated in Fig. 1 comprises a glass bulb 5 having a reentrant stem 5 sealed therein. The stem 6 is provided with a press 7 in which are sealed support wires 8, 9, in, H and ii. To

the upper ends of wires 9 and ill a mica disc 35 3 quarter cylinders l5 and i5 and wings l8 and i9 may be formed from a continuous sheet of substantially uniform thickness. The free edges of Wings 58 and i9 are bent partially around support wires 9 and I0, respectively, and welded thereto. Anode wires and M are welded at one end to support wires ii and i2, respectively, and held in place at their other ends by passing through eyelets in mica disc 35. These anode wires 20 and 21 are positioned in the front of 5 are of molybdenum approximately 10 mils in diameter. The space between the convex surfaces of cylinders I and I6 is bisected by a mica sheet 22. Another mica sheet 23 liesagainst the inner faces of wings I6 and I9 of cathode I4. These two sheets 22 and 23 are substantially at right angles to one another and interconnected at slots 24 and 25. The sheet 22 bears against one side of support rod l3. A nickel baflle 26 is supported from the other end of bent wire I3 to which the battle 26 is welded. A metallic cup 21 is supported from baiiie 26 by means of wire 28. A mixture for producing caesium vapor when heated is carried within the cup 21, the mixture being held in place by metallic gauze 29. The stem is provided with an exhaust tube 30.

A coil 3| is arranged to induce high frequency current in the metal cup 21 for heating the mixture within the cup to its reaction temperature. This coil 3| is used only during the processing of the tube.

Sealed in the press I are lead-in wires 32, 33 and 34 connected respectively to support wires 9. II and I2. Consequently, lead-in wire 32 is conductively connected to cathode I4. Lead-in wires 33 and 34 are conductively connected respectively to anodes and 2i.

A portion of the glass bulb 5 is shown broken away in Fig. 1 for clearness of illustration.

The cross-sectional shape of the cathode I4 and location of the anode wires 2ii and 2| are more clearly shown in Fig. 2 which is a cross-section through the tube at the largest diameter of the bulb 5 perpendicular to the axes of the quar-,

ter cylinders I5 and I6 looking toward the mica disc 35. Due to the configuration of the cathode I4 and the location of the anodes 20 and 2|, substantially all of the electrons emitted from either portions I5 or I 6 of the cathode I4 will be collected by the corresponding anode. In view of this fact, the efiiciency of the tube when used in the so-called push-pull projection circuit is very high.

The space within the cathode l4 bounded by the convex faces of quarter cylinders I5 and I6 and the support wings I8 and I3 is divided by mica insulators 22 and 23 so that metal does not face metal. Due to such division of this space by insulators there is no tendency for the discharge between the cathode I4 and the anodes 20 and 2i to concentrate on the inside surfaces of the cathode I4 as would be the case were the mica sheets omitted.

The shape of the formed cathode I4 and the mica sheets 22 and 23 are clearly shown in the exploded view of Fig. 3. When assembled sheet 23 lies against the inner faces of support wings l3 and I9, the unslotted portion of sheet 23 lies within the slot 24 of sheet 22 and the unslotted portion of sheet 22 lies within the slot 25 of sheet 23.

The electrodes and accessory structure carried by the stem 6 are fabricated before the stem is sealed into the glass bulb 5. Cathode I4 is formed from a polished silver sheet of a high degree of purity which has a clean mirror finish. The formed cathode is washed and reduced in hydrogen. The concave surfaces of quarter cylchromic oxide Cl'aOa, and 9 milligrams of powdered aluminum Al. These ingredients are carefully prepared, finely pulverized and thoroughly mixed in the proper proportions before being formed into pellets.

The pumping station illustrated in Fig. 4 is adapted for the processing of four tubes concur rently. The tubes 5 are sealed to a glass header 40 by means of exhaust tubes 30. This header 40 runs through an oven 4| which comprises a base 42 carrying end supports 43 on the upper ends of which is a metallic cap 44. An electric heater paratus comprising a liquid air trap 49, two Mc- Leod gauges and 5|, mercury vapor pump 52 and mercury cut-off 53, and a vacuum pump (not shown) connected to tube 54.

Between the mercury cut-oil 53 and the liquid air trap 49, two gas supply units 55 and 56 are connected to the pumping apparatus. These units comprise flexible coiled glass tubes 5I associated with gas containers 58 and 55 through mercury seals surrounding pairs of porous plugs of Lavite," which plugs when brought together permit gas to pass from the respective containers 58 and 59 to the bulbs 5 through the glass coils 51. An ionization manometer 60 is connected to the other end of the header 4!).

An electricalcircuit used during the processing of four tubes while they are sealed on a pumping station is illustrated in Fig. 5. This circuit comprises conductors I0 adapted to be connected to a direct current source (not shown) and a potentiometer II for determining the potentials to be applied between cathodes I4 and anodes 20 and 2| of tubes 5, said anodes being connected in parallel. With switch I2 in its lower position and jack I5 closed, a potential may be impressed on any one of tubes 5 by closing the lower contacts of switch I3 which is individual to such tube. With switch I2 in its upper position, the potential determined by potentiometer II is impressed through the upper contacts of all of switches I3 in series on condenser I4 which have a total capacitance of 5 microfarads. Condenser 14 can then be discharged through any of tubes 5 by closing the lower contacts of the corresponding 1 switch I3. The voltage determined by potenti-.

ometer II is indicated by voltmeter I6. The current flowing through tubes 5 during a later stage of the processing is indicated by ammeter I8 which may be connected into the circuit by inserting plug 1! into jack I5.

The processing of four tubes after the .untreated assemblies have been sealed onto the header 40 will now be described.

The tubes 5 are sealed on to the header 40 with the concave sides of the cathode plates I4 facing directly forward, with the bulbs 5 centered with respect to the high frequency coils 3| and at such a height that the cup 21 is approximately midway of the coil 3| longitudinally.

The vacuum pump is turned on. Liquid air trap 49 is cooled with liquid air. When the pressure has fallen suficiently the manometer 8B is turned on. The heater t5 wlthbut the cover 58 and chimneys M is raised to engage the cap it. Current is turned on to bring the oven temperature up to 400 centlgrade and maintained until the manometer shows a pressure of 5x10- millimeters of mercury, or lower, when the current is shut oif and the oven allowed to cool. Slow leaks anywhere in the system are indicated if the pressure is not less than 2x 10 millimeters of mercury when the tubes are cooled to room temperature. If leaks are indicated they should be eliminated before the processing proceeds. This treatment removes occluded gases from the bulbs but does not cause any chemical reaction in the caesium pellet. The pumping station is then flushedwith oxygen from gas supply unit 5d until a clear green haze is obtained in the manometer 60 which indicates suficient purity of the oxygen.

Mercury cut-off 53 is now closed to cut ed the vacuum pumps and the manometer til turned oif. Additional oxygen is admitted to a pressure of 2.2 millimeters of mercury as indicated by McLeod gauge 50. Switch 112 of the electrical circuit of Fig. 5 is set in its lower position and the potentiometer ll adjusted so that the voltmeter l6 shows a voltage of 700 volts. Switches 13 are then operated in succession to close their lower contacts so that each cathode it in succession is subjected to positive ion bombardment. Due to the insulation between the convex surfaces of the cathode it the bombardment takes place substantially entirely on the concave surfaces of cathode id. The first efiect of such bombardment is to produce a heavy layer of silver oxide on the concave surfaces of cathode it, followed by a heating-up of the silver plate and reduction of the layer of silver oxide. The cathodes are allowed to cool, the omgen pressure again adjusted to 2.2 millimeters of mercury and the oxidation, reduction and cooling repeated.

Bil

The mercury cut-off 53 is then opened and the oxygen pumped out. This treatment leaves the concave surfaces l5 and it of each cathode it clean and slightly rough so that it has a uniform matte finish.

After a good vacuum has again been obtained as indicated by manometer til, the mercury cutoff 53 is again closed and the manometer turned off. A fresh charge of oxygen through gas supply unit 55 is admitted to a pressure of 0.75 millimeter of mercury. Switch 72 is thrown to its upper position after potentiometer ll has been adjusted so that the voltmeter 16 shows a voltage of 650 volts. Each tube 5 is then subjected to a series of discharges from condenser 14 by the operationof closing switches '13 on their lower contacts. This operation is hereinafter called tapping. Switch I? in its upper position is therefore said to be in the tapping position.

Each switch it is tapped sixty times in three groups of twenty taps each. The first group of taps is applied to the four tubes in succession, then the second group and so on until each tube has received the required number of taps or discharges. The mercury cut-ofi 53 is again opened and the pumping continued until the pressure is down to 3X10T'5 millimeters of mercury or less.

This treatment with oxygen by tapping produces a thin layer of silver oxide on the concave surfaces of each cathode it. At each tapping of a switch 13 a glow discharge occurs in the associated tube 5 and the cathode id of that tube is bombarded with a definite number of positive ions. The amount ,of silver oxide which is formed is therefore closely controlled because the discharge is conflned to the concave surfaces l5 and IS.

The cathodes it are now in condition to be treated with caesium vapor under suitable temperature conditions. A source of high frequency current (not shown) is connected to calls 8! in succession to "flash the caesium pellets, that is, to induce sufilclent current in the cups 2? to start a chemical reaction of the ingredients of the caesium pellet. The exothermic reaction which follows develops a large amount of heat and causes the immediate and complete expulsion of all of the caesium. The high frequency source is disconnected as soon as the reaction starts. The caesium, as it travels from the open end of cup 21 through metal gauze 29, is deflected by baille 26 and is condensed on the glass walls of the bulb 5 in front of the oxidized surface of the cathode Id. The bafile 26 prevents the hot caesium vapor from impinging directly on the cathode surface. This baiile 26 is so shaped that it lies substantially parallel to the magnetic lines of force produced by the high frequency current in coil 3| and therefore is not heated to any great extent by eddy currents induced therein.

Immediately after the caesium pellet has been flashed stem heaters (not shown) are inserted around the stems of the tubes 5. Thermocouples are placed against each bulb 5 at the surface opposite'the stem to indicate'temperatures. The cover 4% with the chimneys M are placed on the heater 45 and the whole raised until a chimney d1 surrounds each bulb 5. The stem heaters are turned on first and after two minutes the heater 65 is turned on to quickly bring up the temperature of the convection air currents flowing past the bulbs 5. When the hottest bulb reaches a temperature of 150 centigrade the heater 45 is regulated to hold this temperature for five minutes. At the end of this five-minute period the heater 35 is again regulated to raise the temperature of the air currents to quickly bring the temperature of the hottest bulb 5 up to 225 centigrade. The heater at is then regulated to hold this temperature until the cathode surface of each tube has reached the proper sensitivity.

The sensitivity may be indicated by the ammeter 18 when connected into the circuit of Fig. 5 by the insertion of plug ll into jack #5. Each cathode M may be illuminated by a source of light (not shown). The potentiometer H is adjusted to give a voltage of 50 volts, as shown on voltmeter '86. Switch 72 is closed in its lower position. The sensitivity of each tube 5 will be indicated by the ammeter 78 if the corresponding switch 73 is closed in its lower position. The tubes 5 are tested from time to time while illuminated and the hot air treatment is continued until the photoelectric current tends to decrease. As each tube reaches its proper sensitivity the air current for that tube is cut off. When all of the tubes have reached the proper sensitivity the chimneys 4W are lowered and the bulbs allowed to cool to room temperature with the pumping apparatus still on.

When the bulbs 5 have cooled and the pressure is down to 2x 10- millimeters of mercury, mercury cut-ofi 53 is closed and argon is admitted from the gas supply unit 5d. The argon is admitted until the pressure assumes a steady state at the desired value, when the tube is sealed off from the header it by sealing the exhaust tube 30. The pressure of argon is dependent upon the amount of gas amplification desired in the completed tube.- A suitable pressure is 5x10- millimeters of mercury.

The cathode I4 may comprise 'a nickel or copper sheet having a surface layer of silver which is processed in the manner hereinbefore described to render it photosensitive. The preliminary roughening of the silver surface may be accomplished in other ways than by oxidation and reduction of the silver oxide, as, for example, by sandblasting, etching with acid, or even by applying a coating of silver oxide and a binder with a brush or air spray and reducing the silver oxide to metallic silver by heat.

The cathodes may also be sensitized by subjecting the oxidized silver surface to the vapor of the other alkali metals, rubidium, lithium, potassium and sodium. The temperature conditions would be varied somewhat depending upon the materials being used.

What is claimed is:

1. A photoelectric tube comprising an evacuated container, a metallic cathode plate comprising two concave quarter-cylindrical surfaces of quarter circular cross-section joined along one edgeand curved away from each other so that a straight line joining any point within the quadrant of one surface with any point within the quadrant of the other surface intersects both surfaces, metallic means extending from the outer edges of said cylinders toward each other, means supporting said cathode within said container, two anodes also supported within said container in front of said cathode surfaces respectively and parallel thereto lying in a plane intersecting both cathode surfaces intermediate said anodes, a sheet of insulating material extending from the joined edges of said cathode to a point between said metallic means which extends from the outer edges, and a light sensitive electric layer on both said concave surfaces.

2. A photoelectric tube comprising a cathode composed of sheet metal of approximately uniform thickness formed with portions of one side of the sheet turned toward each other and portions of the other side of the sheet turned away from each other, insulation in the space between the portions turned toward each other, an anode in the space in front of the portions turned away from each other, and means mounting said cathode, anode and insulation in said specified relationship in an evacuated container.

3. A photoelectric tube comprising an evacuated container, a cathodeelement in the form of a. hollow open-ended member whose outer surface is a nearly closed cylinder having a crosssection resembling an arrowhead, a pair of anodes outside said cathode element, and a. sheet of insulation bisecting longitudinally the space within said cathode element.

4. A photoelectric tube comprising an evacuated container, a metallic cathode plate com prising two concave quarter-cylindrical surface of quarter circular cross-section joined along one edge and curved away from each other so that a straight line joining any point within the quadrant of one surface with any point within the quadrant of the other surface intersects both surfaces, means supporting said cathode plate within said container, two anodes also supported within said container in front of said concave surfaces respectively and parallel thereto lying in a plane intersecting both cathode surfaces intermediate said anodes, a sheet of insulating material between the cylindrical surfaces of said cathode extending from the joined edges of said cathode to a plane passing through the remote edges of said cathode, and a light sensitive electric layer on both said concave surfaces.

5. A photoelectric tube comprising a light pervious envelope with a reentrant stem supporting side by side two parallel sheet-like photoelectric cathodes sensitized with light responsive material on one surface and with adjacent edges joined together and with the perpendicular planes bisecting the light sensitive surfaces intersecting each other on a line behind the light sensitive surfaces, electron collecting means in front of said light sensitive surfaces, and a sheet of insulating material extending from said joined edges to a plane passing through the outer edges of said cathodes.

6. A photoelectric tube comprisinga container having therein a cathode the rear surface of which comprises portions generally convergent toward a central portion, an anode structure near the front surface of said cathode, a gas filling of such density that a glow discharge takes place between said cathode and anode when a suitable potential is applied therebetween, said glow discharge taking place partly between said anode and the rear surface of said cathode when there is a clear space between said convergent portions,

and means in said space offering impedance to the ionic flow to or from said surface to cause the discharge to be predominantly or wholly within the region between the front surface of said cathode and said anode structure.

SYDlW B. INGRAM. 

